Yale paleontologist Derek Briggs, a co-author of the study, said the exceptionally well-preserved fossil, which is less than 10 mm long, revealed not only the animal’s hard shell but also its limbs, eyes, gut, and gills. Briggs said the animal’s respiratory system included five pairs of gills with canals that conveyed essential fluids — implying that a heart evolved in representatives of ostracods, this common group of micro-crustaceans, more than 430 million years ago.

The first author of the study is David Siveter of the University of Leicester.